Pump

ABSTRACT

A fluid pump comprising a flow channel containing an fluid inlet and a fluid outlet and bounded by two side walls, a substantially planar flap positioned inside the flow channel, and an actuator capable of transmitting an oscillating force or torque to the flap, where the side walls extend from the inlet to the outlet and are substantially planar and parallel to the flap and extend beyond the downstream end of the flap towards the outlet by a distance such that l d ≧l f /2, where l f  is the length of the flap, where the side wall separation, h, length, l w , and width, w w , satisfy the relationships: l w &gt; h  and w w &gt;h, whereby in use, the actuator drives oscillatory motion of the flap in a direction substantially perpendicular to the side walls with motion of the flap having larger amplitude near the outlet than near the inlet.

This invention relates to a pump in which fluid is propelled by theoscillating motion of a flap. The flap is contained within a flowchannel with side walls substantially parallel to the flap. In use, theflap motion generates a series of counter-rotating vortices whichinteract with the side walls, with the flap and with each other togenerate a fluid flow. The vortices also have a mixing function and thepump can be used to exchange heat between the fluid being pumped and theside walls. The pump can also be used as a mixer to combine two inletfluid flows to form a well-mixed outlet fluid flow.

We use the term fluid to refer to both gases and liquids. We use theterm pump to refer to a device to create a flow of a fluid from an inletto an outlet where the outlet pressure is higher than the inletpressure, including liquid pumps, air pumps and air fans.

Rotating fans and pumps are well known for pumping gases and liquids.However the efficiency of these pumps tends to decrease as their sizebecomes small (typical dimensions of less than 5 cm), due to motorlosses, bearing friction, viscous drag and blade tip leakage. The use ofa rotating mechanism requires a bearing which may need lubrication, havea limited lifetime, or be vulnerable to dust.

Rotating fans and pumps are not well suited to generating in-plane fluidflow in thin devices, as axial flow fans and pumps generate flowperpendicular to their plane of rotation, and centrifugal fans and pumpsrequire an axial inlet flow and tangential outlet flow. Therefore it isdifficult to package rotating fans in a thin format suitable for laptopcomputers, portable electronic devices, and heat exchangers forsemiconductor devices.

Rotating fans and pumps are often used to provide a fluid flow through aheat exchanger containing a set of heated or cooled fins. Use ofseparate fluid moving and heat exchange parts requires additional spaceand leads to reduced heat exchange performance by not making use ofrotational fluid flows generated by the pump or fan to enhance thermalmixing and heat exchange performance.

Rotating fans and pumps usually have rotation speeds and blade passingfrequencies in the audible range (100 Hz to 20 kHz), generating periodicnoise. Rotating fans and pumps may also require high blade tipvelocities (often greater than 10 ms), generating noise with a broadfrequency spectrum. These noise sources can be undesirable in manysituations.

Oscillating flap fans and pumps are known, in particular piezoelectricfans which often operate at frequencies of 50 Hz or 60 Hz. The lowfrequency requires a large amplitude of motion to achieve flapvelocities of >1 ms which are typically required to generate significantflow. The large amplitude of motion limits use in thin devices.Currently known piezoelectric fans may be combined with a heat exchangerbut are not optimised in choice of flow channel geometry surrounding theflap, oscillation frequency and amplitude, and use of aerodynamic flapprofiles. This combination of factors results in currently knownpiezoelectric fans generating relatively weak fluid flows, in particularwith low stall pressure.

The aim of the present invention is to overcome the disadvantages of thefans and pumps described above.

This invention relates to a fluid pump in which a substantially planarflap is positioned within a flow channel having an inlet and an outletand bounded by two side walls. The side walls extend from the inlet tothe outlet and are substantially planar and parallel to the flap. Theheight of the flow channel, h, is defined by the separation of the sidewalls.

The flow channel may also be bounded by a second pair of wallsperpendicular to the side walls and parallel to the flow direction. Weuse the term edge walls to refer to this second pair of walls. The edgewalls extend along the length of the flow channel from the inlet to theoutlet. Preferably the flap extends across the entire width of the flowchannel, save for a small gap to avoid contact between the flap and theedge walls.

The flap and flow channel may have several forms: rectangular, sectorannular where the sector angle is less than 360°, or full (360°)annular. In the sector annular and full annular cases, the direction offluid flow is in a radial direction and the lengths of the flap, sidewalls, edge walls and flow channel mean their respective dimensions in aradial direction and the widths of the flap, side walls and flow channelmean their dimensions in a circumferential direction.

The flow channel has height h and is bounded by side walls with lengthl_(w) and width w_(w) and separation h, where l_(w)>h and w_(w)>h.

The flap has length l_(f) in the direction parallel to the fluid flowand width w_(f) in the direction perpendicular to the fluid flow suchthat w_(f)>h and preferably l_(f)>2h and w_(f)>2h. In the case of a flaphaving a sector annular or full annular form, the width of the flapw_(f) is taken to be the length of the edge of the flap nearest theoutlet, taken along a circumferential path.

The pump may exploit a geometric flow velocity amplification effect inwhich the ratio of fluid flow velocity to flap velocity increases inproportion to l_(f)/h, so it is preferable to increase the ratio l_(f)/hin order to increase pump performance.

It is also preferable to minimise fluid flows in directionsperpendicular to the flow direction as these are wasteful and may reducethe pump performance and efficiency. These perpendicular flows may occurbetween the sides of the flap and the edge walls of the flow channel,and their negative impact on pump performance and efficiency can bereduced by increasing the ratio of the flap width w_(f) to the flowchannel height h such that w_(f)>2h. It follows that the flow channelwidth which is wider than the flap it encloses is also substantiallygreater than the flow channel height.

In order to generate a strong flow and pressure rise, it is importantthat the side walls extend downstream of the flap by a distance l_(d)where l_(d)>l_(f)/2 and preferably l_(d)>2h. Within the length l_(d)downstream of the flap the side walls are continuous and the flowchannel between the side walls is substantially free from additionalstructures. The substantially unobstructed flow channel downstream ofthe flap is required to allow space for interactions of vortices witheach other and with the side walls. These interactions generate apressure rise downstream of the flap and increase the pump performance.

Individually some of these features are known in the prior art:

U.S. Pat. No. 4,498,851 describes oscillating flaps to generate a fluidflow.

U.S. Pat. No. 4,923,000 shows walls parallel to flaps but not extendingdownstream of the flap.

U.S. Pat. No. 5,861,703 shows walls parallel to flap but not extendingsignificantly downstream of the flap.

U.S. Pat. No. 7,321,184 shows walls perpendicular rather than parallelto the flap.

U.S. Pat. No. 4,834,619 shows walls downstream of the flap and parallelto the flap but not forming a flow channel surrounding the flap.

FR2528500A1 shows an oscillating flap in a flow channel, but the flowchannel does not have an unobstructed region downstream of the flap.

JP2002339900A shows an oscillating flap in a flow channel, but thedownstream region of the flow channel contains additional structures andwhich form smaller channels not satisfying the condition that channelwidth is substantially greater than channel height. US20110064594A1 alsocites the design described in JP2002339900A as an example of prior art.

JPH0312493U shows an oscillating fan in a channel with squarecross-section, while the current invention requires a flow channel andflap with width greater than the height. Additionally, JPH0312493U showsinlets beside the flap while the current invention requires side wallsin this region.

U.S. Pat. No. 5,941,694A shows multiple flaps in a flow channel butthese flaps to not have an unobstructed region of flow channel extendingdownstream by more than twice the side wall separation.

A flow channel with an unobstructed region immediately downstream of theflap is required to provide a space for interaction of vortices witheach other and with the side walls to provide a pressure rise downstreamof the flap and thereby to improve the pump performance. In thisinvention we describe a combination of geometry of flow channel andgeometry and motion of oscillating flap that is required for high pumpperformance, and this combination is not known in the prior art.

A further benefit of the unobstructed region of flow channel downstreamof the flap is to provide space for the vortices to mix the fluid, whichis useful in the case where the pump acts as a mixer or heat exchanger.

The pump is equipped with an actuator which provides an oscillatingforce or torque to drive oscillatory motion of the flap.

In use, the direction of flap motion is substantially perpendicular tothe side walls and the motion of the flap has larger amplitude near theoutlet than near the inlet, causing the flap to create and shed vorticesin the fluid being pumped, with interaction of the vortices with eachother, with the flap and with the side walls creating a fluid flow andpressure rise downstream of the flap. The side walls contain the vortexstreet generated by the flap oscillation and increase the fluid flow andpressure, compared to a piezoelectric fan not provided with side wallsof the geometry shown in FIG. 1.

The mechanism of generating fluid movement for propulsion by oscillatingmotion of flaps or aerofoils is well known in nature and is used by fishand birds for swimming and flying. This mechanism has also beeninvestigated for ship propulsion and for micro-aerial vehicles. In thepresent invention, the flapping propulsion mechanism is enhanced byproviding static side walls that extend downstream of the oscillatingflap.

A qualitative explanation of the flow generation mechanism is givenbelow, in terms of vortices generated by the flap and their interactionswith the side walls. The side walls can be conceptually replaced byimage line and sheet vortices. The image line vortices have the oppositesense of rotation to the real vortices in the flow channel, such thatthe wall-perpendicular velocity components of a real and image vortexpair sum to zero. The image sheet vortices at the wall locations providezero slip at the walls. These vortex sheets occur in pairs separated bystagnation points at the wall. The sheet vortices exert shear forces onthe fluid in the flow channel and diffuse into the flow channel at arate depending on the fluid viscosity. The net effect of the sheetvortices is to exert a downstream force on the fluid, causing fluid inthe flow channel to move from the inlet to the outlet.

The cross-section of the flap and flow channel perpendicular to thewidth direction may be substantially uniform across the width of thepump, so that different designs with increased or decreased width andflow rate can be created easily and can share common manufacturingprocesses.

The flap may have an aerodynamic or aerofoil shape or a thin trailingedge to enhance vortex formation and shedding and to reduce drag.

There may be a piezoelectric or magnetostrictive bending actuatorincorporated into or mounted on the flap.

The flap may be driven by a remote actuator using a mechanicalconnection or a hydraulic or pneumatic drive.

The flap may be driven by electrostatic or magnetic forces.

The flow channel inlet may be divided into two regions to combine twofluid inlet streams, such that in use, the motion of the flap generatesvortices and causes the two inlet fluid streams to be pumped and mixeddownstream of the flap.

There may be a temperature difference applied between one or both sidewalls and inlet fluid stream, such that in use, the motion of the flapgenerates vortices causing the inlet fluid stream to be pumped and toexchange heat with one or both side walls, with the circulating flow ofthe vortices enhancing heat transfer. The motion of the flap may bedriven at ultrasonic frequencies (>20 kHz) to provide operationinaudible to humans.

The motion of the flap may be driven at low frequencies (<400 Hz), belowthe frequency of peak sensitivity of the human ear, to provide quietoperation

The flap may have maximum peak-to-peak displacement, A, between 10% and70% of the side wall separation. In any case, it is preferable that theflap does not impact the side walls during operation.

The flap oscillation frequency, f, may be chosen to give a Strouhalnumber, St=f A/U between 0.1 and 0.5, where U is the average fluid flowspeed in the flow channel. A Strouhal number in this range is found toprovide efficient propulsion for a wide range of swimming and flyinganimals.

The amplitude of flap motion may be amplified by mechanical resonance ofthe flap.

The flap may be clamped at the edge near the inlet. Alternatively theflap may oscillate with fixed centre of mass and be supported by twopivot supports at nodal locations, or the flap may be supported by aflexible vibration isolating support.

An electromechanical actuator mounted on the flap may be provided withelectrical connections using flexible support wires, or by a flexiblecircuit acting as a vibration isolating support.

The flap may have a flexible construction such that fluid loading causesnon-sinusoidal motion of the flap.

The pump may contain two or more flaps, where the flaps move with out ofphase motion to avoid noise and vibration.

The flap may be fabricated from a folded sheet metal structure with alaser-welded seam.

The pump may consist of an array of oscillating flaps contained withinflow channels. A single actuator may drive multiple flaps.

The pump may contain an array of multiple flaps fabricated from a singlesheet.

The pump may contain an array of flaps supported by a common supportframe.

DESCRIPTION OF THE FIGURES

FIG. 1 shows a fluid pump 1 comprising a flow channel 10 with an inlet 2and outlet 3. The flow channel is bounded by side walls 4 and edge walls6. The flow channel contains a flap 8 with actuator 9 attached to theflap. The flap has length l_(f) and width w_(f) where preferablyw_(f)>2h. The side walls and flow channel extend downstream of the flapby a distance l_(d), where l_(d)≧l_(f)/2 and preferably l_(d)>2h. It ispreferable for the flap length l_(f) to satisfy the condition l_(f)>2h.The side walls have length l_(w) and width ww and satisfy therelationships l_(w)>h and w_(w)>h.

FIG. 2 shows a plan view of the pump and flow channel, showing inlet 2,outlet 3, edge walls 6, flap 8 and actuator 9 attached to the flap.

FIG. 3A shows a side view of the pump and flow channel, showing inlet 2,outlet 3, side walls 4, and flap 8. The flap oscillates towards the sidewalls and the extreme positions of the flap are shown by dashed lines.The motion of the flap is larger at the end nearer the outlet 3 and thismotion generates a series of counter-rotating vortices 5 forming areverse von Kárman vortex street. In an inviscid approximation, the sidewalls can be replaced by image vortices 12 with opposite sense ofrotation to the real vortices 5. FIG. 3B shows the variation of pressurewith position along the flow channel. There is a pressure rise frominlet to outlet, indicating fluid pumping function. A substantial partof the pressure rise occurs downstream of the flap due to theinteraction of the vortices with the side walls and due to theinteractions of alternating vortex pairs which form jet flows orienteddownstream.

FIG. 4 shows three forms of the pump in plan and section views, with thefluid flow directions indicated by arrows. FIG. 4A show a rectangularform pump. FIG. 4B shows a sector annular form pump, with the sametopology as the rectangular form pump but formed into a curved shape.FIG. 4C shows a full annular form pump in which the air flows radiallyoutwards. It is also possible to create an annular pump in which theinlet and outlet locations are reversed with respect to those shown inFIGS. 4B and 4C, and the flap is arranged so as to generate a radialflow travelling inwards towards the inner edge of the pump.

FIG. 5A shows a von Kárman street created by flow past a cylinder. Thebluff body generates a drag wake composed of staggered counter-rotatingvortices with interspersed jet flow oriented upstream. FIG. 5B shows astreamlined foil which generates a reverse von Kárman street. Thisactively generated wake produces jet flow between alternating vortexpairs that is oriented downstream (after INTEG. AND COMP. BIOL.,42:243-257 (2002)).

FIGS. 6A and 6B illustrate the fluid flow in the flow channel at twopoints in time. Fluid velocity is indicated by arrows and contours ofmagnitude of vorticity are indicated by solid lines. The flap is alsoshown. FIG. 6B illustrates the fluid flow and flap position at timeapproximately one quarter period of flap oscillation later than FIG. 6A.Each vortex generates a vortex sheet at the wall to counter slipvelocity induced by an image vortex of the type described in FIG. 3.Generation of a new vortex pulls the previous vortex over to same side,so the resulting vortex pair generates a pair of wall sheet vortices ofopposing senses separated by a stagnation point. The pair of sheetvortices exert shear forces on the fluid with opposite directions, butthe downstream shear forces dominate creating a net downstream force onthe fluid and generating a fluid flow.

FIG. 7A shows fluid motion generated by an oscillating flap 8.Time-averaged fluid flow is from left to right and vortices 5 generatedby the oscillating flap are shown downstream of the flap. FIG. 7B showstemperature contours 31 in the fluid flow generated by an oscillatingflap in the case where there is a temperature difference between heatedor cooled side walls 29 and the fluid at the inlet 2. Upstream of theflap a boundary layer 30 grows in thickness, slowing heat transfer.Downstream of the flap, vortices disrupt the boundary layer and speed upheat transfer.

FIG. 8A shows the cross-sectional shape of an aerodynamically shapedflap 8. FIG. 8B shows a similarly shaped flap containing a bendingactuator containing a piezoelectric or magnetostrictive layer 13 and anelastic layer 14.

The aerodynamic shape can be created by folding a sheet of material 16and joining the sheet to itself at a line 15 located between the bendingactuator and the downstream end of the flap.

FIG. 9 shows two oscillating fans 8 driven by a single actuated rod 17connected to each fan by a pivot support 18. Clamps 19 prevent movementat one end of the flaps, while the couple generated by the combinationof clamping force and driving force causes motion at the other end ofthe flaps.

FIG. 10 shows a pump with two inlets 20, 21 and one outlet 3. Flow isdriven from the inlets to the outlet by an oscillating fan 8 which alsocauses the inlet fluid flows to become mixed by the vortex-rich flowgenerated by the oscillating fan.

FIG. 11A shows an oscillating fan 8 supported by a clamp 19 at one endand oscillating in a fundamental bending mode at the other end. FIG. 11B shows an oscillating fan 8 vibrating about its centre of mass,supported by two pivot supports 23 which do not constrain the angle ofthe fan, and with a mass 22 attached to one end of the fan such that theamplitude of motion at the other end of the fan is larger.

FIG. 12 shows a heat exchanger with inlet fluid flow 25 driven byoscillating fans 8 between static walls 4. The static walls are inthermal contact with pipes 24. The pipes may contain a pumpedcirculating flow of heat-carrying fluid, or they may be heat pipescontaining liquid and vapour transported by evaporation and condensationprocesses and capillary forces, or they may be solid conductors. FIG.12A shows a side view and FIG. 12B shows a plan view.

FIG. 13 shows a heat exchanger 28 with integrated array of fans 8 andstatic walls 4 which also serve as heat sink fins, conducting heat to orfrom the base of the heat exchanger 27. A fluid flows from the inlet 2to the outlet 3. When used as a heat sink, heat flows from the fins tothe fluid. When used as a device to cool a fluid, heat flows from thefluid to the fins. The motion of the fins 8 is indicated by dotted linesand is substantially perpendicular to the static walls.

FIG. 14 shows load curves for an oscillating fan device driven atvoltages from 40 Vpp to 140 Vpp using a piezoelectric bimorph bendingactuator oscillating at approximately 250 Hz. In this case fluid beingpumped is air and the approximate dimensions of the device are:

side wall separation, h:  9 mm flap width, w_(f): 63 mm flap length,l_(f): 31 mm side wall width, w_(w): 64 mm total side wall length,l_(w): 80 mm downstream length, l_(d): 49 mm

FIG. 15 compares the heat transfer provided by the oscillating fan withthe heat transfer provided by a conventional rotating fan, as a functionof air flow rate through a heat exchanger. The oscillating fan providessignificantly greater the heat transfer than a rotating fan operating atthe same air flow rate.

FIGS. 16, 17, 18, 19, 20 and 21 show examples of the prior art.

1. A fluid pump comprising: a flow channel containing an fluid inlet anda fluid outlet and bounded by two side walls, a substantially planarflap positioned inside the flow channel, and an actuator capable oftransmitting an oscillating force or torque to the flap, where the sidewalls extend from the inlet to the outlet and are substantially planarand parallel to the flap and extend beyond the downstream and of theflap towards the outlet by a distance such that l_(d)≧l_(f)/2, wherel_(f) is the length of the flap, where the side wall separation, h,length l_(w), and width, w_(w), satisfy the relationships: l_(w)>h andw_(w)>h, whereby in use, the actuator drives oscillatory motion of theflap in a direction substantially perpendicular to the side walls withmotion of the flap having larger amplitude near the outlet than near theinlet.
 2. A pump according to claim 1, where the region of flow channelbetween the flap and the outlet is substantially unobstructed.
 3. A pumpaccording to claim 1, wherein the flap length l_(f) satisfies thecondition l_(f)>2h.
 4. A pump according to claim 1, where the side wallsextend beyond the downstream end of the flap towards the outlet by adistance l_(d) such that l_(d)>2_(h).
 5. A pump according to claim 1,where the where the flap width, w_(f), satisfies the condition w_(f)>2h.6. A pump according to claim 1, where the flap and flow channel havesector annular forms.
 7. A pump according to claim 1, where the flowchannel is also bounded by one or more edge walls, where the edge wallsare perpendicular to the side walls and parallel to the flow direction.8. A pump according to claim 1, where the flap and flow channel havefull annular forms.
 9. A pump according to claim 1, where motion of theflap causes creation and shedding of vortices into the fluid beingpumped, with interaction of the shed vortices with the side walls, witheach other and with the flap creating a pressure rise downstream of theflap.
 10. A pump according to claim 1, where the flap has an aerodynamicor aerofoil shape or a thin trailing edge.
 11. A pump according to claim1, where the flap has substantially uniform cross-section perpendicularto the width direction.
 12. A pump according to claim 1, where apiezoelectric or magnetostrictive bending actuator is incorporated intoor mounted on the flap.
 13. A pump according to claim 1, where the flapis driven by a remote actuator using a mechanical connection or ahydraulic or pneumatic drive.
 14. A pump according to claim 1, where theflap is driven by electrostatic or magnetic forces.
 15. A pump accordingto claim 1, where the flow channel inlet is divided into two regions tocombine two fluid inlet streams and, in use, the motion of the flapgenerates vortices causing the two inlet fluid streams to be pumped andmixed downstream of the flap.
 16. A pump according to claim 1, where atemperature difference is applied between the side walls and inlet fluidstream, such that in use, the motion of the flap generates vorticescausing the inlet fluid stream to be pumped and to exchange heat withone or both side walls.
 17. A pump according to claim 1, where themotion of the flap is driven at ultrasonic frequencies (>20 kHz).
 18. Apump according to claim 1, where the motion of the flap is driven at lowfrequencies (<400 Hz).
 19. A pump according to claim 1, where maximumpeak-to-peak displacement of the flap, A, is between 10% and 70% of theside wall separation, h.
 20. A pump according to claim 1, where theoscillation frequency, f, is chosen to give a Strouhal number, St=f A/Ubetween 0.1 and 0.5, where U is the average fluid flow speed in the flowchannel.
 21. A pump according claim 1, where the amplitude of flapmotion is amplified by mechanical resonance of the flap.
 22. A pumpaccording to claim 1, where the flap is clamped at the edge near theinlet.
 23. A pump according to claim 1, where the flap oscillates withfixed centre of mass and is supported by two pivot supports at nodallocations or is supported by a flexible vibration isolating support. 24.A pump according to claim 1, where an electromechanical actuator mountedon the flap is provided with electrical connections by using flexiblesupport wires or a flexible circuit acting as a vibration isolatingsupport.
 25. A pump according to claim 1, where the flap has flexibleconstruction such that fluid loading causes non-sinusoidal motion. 26.One or more pumps according to claim 1, including a total of two or moreflaps, where the flaps move with out of phase motion to avoid noise andvibration.
 27. A pump according to claim 1, where the flap includes afolded sheet structure with a welded or adhesive bonded seam.
 28. Anarray of pumps according to claim
 1. 29. A heat exchanger containing anarray of devices as claim
 1. 30. An array of devices according to claim1 where a single actuator drives multiple taps.
 31. An array of devicesaccording to claim 1 where multiple flaps are fabricated from a singlesheet.
 32. An array of devices according to claim 1 where multiple flapsare supported in a common support frame.